2 :mod:`asynchat` --- Asynchronous socket command/response handler
3 ================================================================
6 :synopsis: Support for asynchronous command/response protocols.
7 .. moduleauthor:: Sam Rushing <rushing@nightmare.com>
8 .. sectionauthor:: Steve Holden <sholden@holdenweb.com>
11 This module builds on the :mod:`asyncore` infrastructure, simplifying
12 asynchronous clients and servers and making it easier to handle protocols
13 whose elements are terminated by arbitrary strings, or are of variable length.
14 :mod:`asynchat` defines the abstract class :class:`async_chat` that you
15 subclass, providing implementations of the :meth:`collect_incoming_data` and
16 :meth:`found_terminator` methods. It uses the same asynchronous loop as
17 :mod:`asyncore`, and the two types of channel, :class:`asyncore.dispatcher`
18 and :class:`asynchat.async_chat`, can freely be mixed in the channel map.
19 Typically an :class:`asyncore.dispatcher` server channel generates new
20 :class:`asynchat.async_chat` channel objects as it receives incoming
24 .. class:: async_chat()
26 This class is an abstract subclass of :class:`asyncore.dispatcher`. To make
27 practical use of the code you must subclass :class:`async_chat`, providing
28 meaningful :meth:`collect_incoming_data` and :meth:`found_terminator`
30 The :class:`asyncore.dispatcher` methods can be used, although not all make
31 sense in a message/response context.
33 Like :class:`asyncore.dispatcher`, :class:`async_chat` defines a set of
34 events that are generated by an analysis of socket conditions after a
35 :cfunc:`select` call. Once the polling loop has been started the
36 :class:`async_chat` object's methods are called by the event-processing
37 framework with no action on the part of the programmer.
39 Two class attributes can be modified, to improve performance, or possibly
40 even to conserve memory.
43 .. data:: ac_in_buffer_size
45 The asynchronous input buffer size (default ``4096``).
48 .. data:: ac_out_buffer_size
50 The asynchronous output buffer size (default ``4096``).
52 Unlike :class:`asyncore.dispatcher`, :class:`async_chat` allows you to
53 define a first-in-first-out queue (fifo) of *producers*. A producer need
54 have only one method, :meth:`more`, which should return data to be
55 transmitted on the channel.
56 The producer indicates exhaustion (*i.e.* that it contains no more data) by
57 having its :meth:`more` method return the empty string. At this point the
58 :class:`async_chat` object removes the producer from the fifo and starts
59 using the next producer, if any. When the producer fifo is empty the
60 :meth:`handle_write` method does nothing. You use the channel object's
61 :meth:`set_terminator` method to describe how to recognize the end of, or
62 an important breakpoint in, an incoming transmission from the remote
65 To build a functioning :class:`async_chat` subclass your input methods
66 :meth:`collect_incoming_data` and :meth:`found_terminator` must handle the
67 data that the channel receives asynchronously. The methods are described
71 .. method:: async_chat.close_when_done()
73 Pushes a ``None`` on to the producer fifo. When this producer is popped off
74 the fifo it causes the channel to be closed.
77 .. method:: async_chat.collect_incoming_data(data)
79 Called with *data* holding an arbitrary amount of received data. The
80 default method, which must be overridden, raises a
81 :exc:`NotImplementedError` exception.
84 .. method:: async_chat.discard_buffers()
86 In emergencies this method will discard any data held in the input and/or
87 output buffers and the producer fifo.
90 .. method:: async_chat.found_terminator()
92 Called when the incoming data stream matches the termination condition set
93 by :meth:`set_terminator`. The default method, which must be overridden,
94 raises a :exc:`NotImplementedError` exception. The buffered input data
95 should be available via an instance attribute.
98 .. method:: async_chat.get_terminator()
100 Returns the current terminator for the channel.
103 .. method:: async_chat.push(data)
105 Pushes data on to the channel's fifo to ensure its transmission.
106 This is all you need to do to have the channel write the data out to the
107 network, although it is possible to use your own producers in more complex
108 schemes to implement encryption and chunking, for example.
111 .. method:: async_chat.push_with_producer(producer)
113 Takes a producer object and adds it to the producer fifo associated with
114 the channel. When all currently-pushed producers have been exhausted the
115 channel will consume this producer's data by calling its :meth:`more`
116 method and send the data to the remote endpoint.
119 .. method:: async_chat.set_terminator(term)
121 Sets the terminating condition to be recognized on the channel. ``term``
122 may be any of three types of value, corresponding to three different ways
123 to handle incoming protocol data.
125 +-----------+---------------------------------------------+
126 | term | Description |
127 +===========+=============================================+
128 | *string* | Will call :meth:`found_terminator` when the |
129 | | string is found in the input stream |
130 +-----------+---------------------------------------------+
131 | *integer* | Will call :meth:`found_terminator` when the |
132 | | indicated number of characters have been |
134 +-----------+---------------------------------------------+
135 | ``None`` | The channel continues to collect data |
137 +-----------+---------------------------------------------+
139 Note that any data following the terminator will be available for reading
140 by the channel after :meth:`found_terminator` is called.
143 asynchat - Auxiliary Classes
144 ------------------------------------------
146 .. class:: fifo([list=None])
148 A :class:`fifo` holding data which has been pushed by the application but
149 not yet popped for writing to the channel. A :class:`fifo` is a list used
150 to hold data and/or producers until they are required. If the *list*
151 argument is provided then it should contain producers or data items to be
152 written to the channel.
155 .. method:: is_empty()
157 Returns ``True`` if and only if the fifo is empty.
162 Returns the least-recently :meth:`push`\ ed item from the fifo.
165 .. method:: push(data)
167 Adds the given data (which may be a string or a producer object) to the
173 If the fifo is not empty, returns ``True, first()``, deleting the popped
174 item. Returns ``False, None`` for an empty fifo.
177 .. _asynchat-example:
182 The following partial example shows how HTTP requests can be read with
183 :class:`async_chat`. A web server might create an
184 :class:`http_request_handler` object for each incoming client connection.
185 Notice that initially the channel terminator is set to match the blank line at
186 the end of the HTTP headers, and a flag indicates that the headers are being
189 Once the headers have been read, if the request is of type POST (indicating
190 that further data are present in the input stream) then the
191 ``Content-Length:`` header is used to set a numeric terminator to read the
192 right amount of data from the channel.
194 The :meth:`handle_request` method is called once all relevant input has been
195 marshalled, after setting the channel terminator to ``None`` to ensure that
196 any extraneous data sent by the web client are ignored. ::
198 class http_request_handler(asynchat.async_chat):
200 def __init__(self, sock, addr, sessions, log):
201 asynchat.async_chat.__init__(self, sock=sock)
203 self.sessions = sessions
206 self.set_terminator("\r\n\r\n")
207 self.reading_headers = True
208 self.handling = False
212 def collect_incoming_data(self, data):
213 """Buffer the data"""
214 self.ibuffer.append(data)
216 def found_terminator(self):
217 if self.reading_headers:
218 self.reading_headers = False
219 self.parse_headers("".join(self.ibuffer))
221 if self.op.upper() == "POST":
222 clen = self.headers.getheader("content-length")
223 self.set_terminator(int(clen))
226 self.set_terminator(None)
227 self.handle_request()
228 elif not self.handling:
229 self.set_terminator(None) # browsers sometimes over-send
230 self.cgi_data = parse(self.headers, "".join(self.ibuffer))
233 self.handle_request()